CH. IV. PRECESSION OF THE EQUINOXES. 31 



the sun to the centre of the earth will not meet the equator 

 but a point to the south of it. But now pass the ball on to 

 the next point when neither pole is in shade, and when it is 

 equal day and equal night over the globe (our spring equi- 

 nox), a line now drawn from the sun will fall directly upon 

 the equator, so that the sun's path meets the equator at 

 this point, which is called the equinoctial point. Pass on 

 till the south pole of your ball is in the dark, the sun will 

 now fall directly on a point north of the equator (making our 

 summer solstice). Pass on again to the point of equal day 

 and equal night, and the sun again falls direct on the equa- 

 tor, causing our autumn equinox. Now, if the earth did not 

 make that small circle in space like the top, the sun would 

 always touch the equator at exactly those same points of 

 the earth's orbit or path round the sun; but the effect of 

 that movement is to pull the equator slightly back, so 

 that the points where the ecliptic and the equator cut 

 each other are 5 of seconds more to the west every year, 

 and in this way the equinoxes travel all round the orbit 

 from east to west in 21,000 years. Hipparchus discovered 

 \}ci\% pre-cession (or going forward) of the equinoxes; though 

 he did not know, what Newton afterwards discovered, that 

 it is caused by the sun and moon pulling at the mass of 

 extra matter which is gathered round the equator. 



